The present invention relates to an air conditioning system, in particular for aircraft, comprising at least one heat exchanger, arranged in a ram air duct, for cooling compressed air by means of a fluid, and comprising at least a first and a second air cycle machine which at the compressed-air end are connected to the heat exchanger.
Such air conditioning systems are used for air conditioning, i.e. heating and cooling, of the cabin, for cabin pressurisation as well as for fresh air supply. In particular in aircraft construction it is of special importance that the air conditioning system is small and lightweight as possible. Furthermore, it is essential that the above-mentioned functions of the air conditioning system are carried out both during normal operation and during partial failure of the installation (redundancy).
From the state of the art, two different concepts for this are known, as shown below:
FIG. 1 shows a first embodiment according to the state of the art in which two autonomous installations, which in most cases are identical, are used in order to carry out the required functions. The system according to FIG. 1 ensures that even in the case of total failure of one installation, the second installation still meets the minimum requirements. At the inlet end, hot compressed air (e.g. 200° C. and 3 bar) from the engines or from an auxiliary unit, impinges on each of the installations. The volume of air is regulated by the flow control valve FCV. Compressed air first flows through the preliminary or primary heat exchanger PHX wherein it is pre-cooled to approx. 100° C. In the downstream compressor C, the air is further compressed and subsequently cooled to approx. 40° C. by the main or secondary heat exchanger SHX. Subsequently, the air flows through a water separation cycle which comprises the following components: a reheater REH, condenser CON, and water separator WE. The air which has been dehumidified in this way subsequently flows through the turbine T where it is expanded and during this process is cooled down to approx. −30° C. From the turbine outlet, the air flows through the cold end of the condenser CON and subsequently into a mixing chamber or into the cabin.
In each ram air duct, a fan FAN is arranged which is connected to the compressor C and the turbine T by a mutual shaft. These components constitute the so-called 3-wheel machine. The fan FAN is used to convey ambient air or ram air through the heat exchangers SHX and PHX. In order to improve the throughput in the ram air duct during flight, a fan bypass is provided in which a check valve GCKV 1 is arranged.
Temperature control or control of the cooling performance of the installation is via a valve TCV which makes it possible to bypass the compressor, SHX, turbine, and water separation cycle. Moreover, cooling performance can be varied by way of the volume of ram air by means of flaps (RAIA and RAOA) at the ram air duct inlet and/or at the ram air duct outlet.
If an air cycle machine comprising a turbine T, compressor C and fan FAN fails, the train of this partially defective installation can continue to be used during flight for the conveyance of compressed air at reduced cooling performance. In this arrangement, the second installation, which is still intact, is supported as far as throughput and cooling performance are concerned. Cooling of compressed air in the partially defective installation then only takes place through the ram air heat exchangers PHX/SHX, without expansion in the failed turbine T. If a line should fracture, for example the line from the flow control valve FCV to PHX, or if the flow control valve FCV or a ram air flap does not close properly, this causes total failure of the installation concerned. In this case, the remaining second installation fulfils the minimum requirements (cooling, pressurisation, . . . ).
The size of the installation is predominantly determined by the large components SHX/PHX and ram air duct.
Apart from the system architecture comprising a 3-wheel ACM for each installation, as shown as an example in FIG. 1, other installation concepts are of course also possible, such as for example a 4-wheel ACM for each installation, or two ACMs, arranged in series, for each installation, or motorised ACMs or different dehumidification systems.
However, all these systems share a common feature in that at least two installations that are arranged separately are used to fulfil the redundancy requirements.
FIG. 2 shows an alternative air conditioning system known from EP 0 891 279 B1. In this embodiment, there is duplication of components which are associated with a relatively high failure probability and with significant negative effects of failure on the system, for example the air cycle machine ACM and the flow control valve FCV. In contrast, there is only one (shared) ram air heat exchanger SHX, PHX as well as one water separation system comprising the reheater REH, condenser CON and water separator WE.
Based on the arrangement where there is only one each of the large components PHX, SHX and the ram air duct, the design is relatively compact, and consequently, the space requirements are modest when compared to the system architecture shown in FIG. 1 above.
The fundamental cooling process corresponds to that explained in the context of FIG. 1. During normal operation, hot compressed air from the engines or from an auxiliary unit, e.g 200° C. and 3 bar) flows through the two flow control valves FCV 1, FCV 2. Subsequently, the air is brought together and precooled to approx. 100° C. in the shared primary heat exchanger PHX. Approximately half of the PHX outlet air is compressed in compressor 1 (C1) while the other half is compressed in compressor 2 (C2) and after having been brought together is cooled by the ram air to approx. 40° C. in a secondary heat exchanger SHX.
For condensation and water separation, the cooled compressed air is fed through the reheater REH, condenser CON and the water separator WE. Subsequently, the compressed air which has been dehumidified in this way is divided again and about half each is expanded in turbine 1 (T1) and turbine 2 (T2) respectively, and during this process the compressed air is cooled to −30° C. After the cooled air has been brought together, it is fed through the cold end of the condenser CON and finally, through one or two pipes, fed into the mixing chamber or cabin of the aircraft.
In this embodiment too, 3-wheel machines are provided each of which comprises a turbine, a compressor as well as a fan FAN 1, FAN 2. Turbine performance is used for driving the compressors C1, C2 and the fans FAN 1, FAN 2. The fans FAN 1, FAN 2 are arranged in parallel so that when the vehicle is on the ground, each fan conveys approximately half of the ambient air through the shared PHX and SHX. During flight, the flow of ram air through PHX and SHX primarily occurs due to ram pressure. The ram air is fed via a ram air duct to the SHX and the PHX, the latter being arranged downstream of the ram air flow, and downstream of the PHX the air is drawn in through a shared duct by the two fans FAN 1 and FAN 2. This ram air then flows back to the environment via two separate fan outlet ducts.
Temperature control of the cooling air is by means of two temperature control valves TCV 1, TCV 2 and the ram air duct flaps RRIA, RAOA 1 and RAOA 2. The valves TCV 1 and TCV 2 additionally serve to ensure synchronous operation of the two air cycle machines.
Failure of one of the air cycle machines (ACM) is a typical fault in an installation according to FIG. 2. In this case too, certain minimum requirements concerning the volume of air and the cooling performance have to be ensured. In order to carry out these functions even in the case of a fault, two additional valves SOV 1, SOV 2 are integrated in the respective turbine inlet, and two additional check valves CCKV 1 and CCKV 2 are integrated in the respective compressor inlet.
If for example ACM 1 fails due to a seized shaft, the check valve CCKV 1 prevents the compressed air from flowing back from the operating compressor C 2 by way of compressor C 1 to the inlet end of the compressor C 2, which would result in an ineffective circular flow. The valve SOV 1 is closed so that the air compressed by the compressor C 2 is not ineffectively expanded by way of the idle turbine T 1, but instead is only expanded by way of the functioning turbine T 2. Due to failure of one ACM, the remaining, intact ACM should now convey all the air. However, this is not possible as each ACM, for reasons of weight and size is designed to handle only approx. 50% of the total air volume arising during normal operation. Double this air volume cannot be handled. In order to nevertheless provide the required volume of air and achieve the necessary throughput in the system, a partial bypass of the remaining operating ACM 2 is thus necessary by opening the TCV 2.
Furthermore, it is necessary that during on-the-ground operation (no ram pressure) when the air cycle machine (e.g. ACM 1) has failed, the associated ram air duct outlet flap RAOA 1 be closed, because otherwise the operating fan FAN 2 would draw in the air from the other outlet duct rather than through the ram air heat exchanger. For this reason, the system shown in FIG. 2 requires at least two controllable ram air outlet flaps.
Apart from the architecture shown as an example in FIG. 2, comprising two 3-wheel ACMs for each installation or for each heat exchanger shared in use, other installation concepts are also possible and known, such as for example two 4-wheel ACMs, arranged in parallel, for each installation, or two serially arranged ACMs for each installation, or motorised ACMs, or different dehumidifying systems.
Irrespective of the above, a common factor of all the known systems is the use of at least two air cycle machines for each installation and for each shared heat exchanger so as to meet the redundancy requirements.
The systems shown in FIG. 1 and FIG. 2 are associated with the following disadvantages:
The system according to FIG. 1 has the disadvantage of increased space requirements which results in a reduction of useable space for other aircraft systems or freight. Moreover, two ram air ducts and thus two ram air inlets and ram air outlets including flaps are necessary in the fuselage, with corresponding space requirements and weight.
The embodiment according to FIG. 2 with a single installation where some of the components are duplicated has the following disadvantages. When compared to the embodiment according to FIG. 1, the following additional components are required: two valves (SOV 1 and SOV 2) as well as two check valves (CCKV 1 and CCKV 2). In particular, the reliability of valves is relatively low, consequently they reduce system reliability. System complexity and costs are increased. Failure of an SOV or of a CCKV in closed position leads to complete failure of the respective air cycle machine.
Furthermore, the system according to FIG. 2 requires two ram air outlet ducts, each with a ram air outlet flap in the fuselage, with the associated disadvantages of a large space requirement as well as heavy weight.
Ensuring synchronous operation of the two air cycle machines requires additional control and regulating effort. Failure of an air cycle machine necessitates quick operation of the valves (for example SOVs) so as to ensure proper operation as well as ensuring the functions of pressurisation, ventilation and cooling. Certain components of the system according to FIG. 2 are only provided singly, i.e. there is no duplication; this applies for example to the line from FCV 1 and FCV 2 to the PHX, and/or to the heat exchanger and/or to the water separation system. Failure of just one of these components, such as for example a line fracture, results in total failure of the entire air conditioning system.
If one air cycle machine fails, the cooling performance and throughput in the system is considerably reduced in comparison to those of the embodiment according to FIG. 1. If for example the air cycle machine ACM 1 fails (shaft seizure),the check valve CCKV 1 prevents the compressed air of the operating compressor C 2 from flowing by way of compressor C 1 (ineffective circular flow). SOV 1 is closed so that the air compressed by the compressor C 2 is not ineffectively expanded by way of the idle turbine T 1, but instead is only expanded by way of the functioning turbine T 2. Due to failure of an ACM, the remaining ACM should now convey all the air. However, this is not possible as each ACM is designed to handle only approx. 50% of the total air volume arising (normal operation). An ACM is thus not in a position to handle double the volume of air. As a result of this, the throughput in, and cooling performance of, the installation in the case of a fault is considerably reduced.
Even by overdimensioning the ACMs, i.e. by designing them to handle e.g. 70% instead of 50% of the total throughput, this disadvantage can only be compensated for inadequately because the space requirements and the weight of the ACMs are increased as a result. Essentially, the weight of a component is a function of the throughput.